Carbon isotope geochemistry of the Precambrian Lomagundi carbonate province,Rhodesia

Carbon isotope measurements carried out on 67 dolomite samples from the Middle Precambrian Lomagundi Group (Rhodesia) have yielded a δ¹³C mean of +8.2 ± 2.6%. vs PDB. With the outcrop of these dolomites extending over a distance of almost 300 km, the Lomagundi dolomite faces is likely to represent the largest isotopically anomalous sedimentary carbonate province ever recorded. It is concluded that the anomalous carbonates formed in a closed basin whose δ¹³C level had been substantially increased as a result of a preferential removal (within sedimentary organics) of the light carbon isotope.     

Coupling among the terrestrial sulfur,carbon and oxygen cycles: numerical modeling based on revised Phanerozoic carbon isotope record

Numerical modeling of the terrestrial oxygen budget based on the revised δ¹³C_carb record by Veizeret al. (1980) has shown that total photosynthetic oxygen has varied between ±7% and ±10% of its average reservoir size (～3.2 × 10²² g) during the last 800 myr as a result of oscillations of the sedimentary reservoir of organic carbon. Calculated curves of oxygen evolution display a distinct minimum in the Early Paleozoic framed by two maxima in the Latest Proterozoic and the Mesozoic. The sympathetic relationship observed between the curves of total oxygen evolution and respective functions for the partial reservoir of sulfate-bound oxygen suggests that the O₂ required for an additional conversion of sulfide to sulfate was most probably provided by excess burial of organic carbon, the results of the modeling thus adding credence to current interpretations proposed for the negative correlation between the secular ${}^{13}\text{C}{}^{12}\text{C}$ and ${}^{34}\text{S}{}^{32}\text{S}$ trends.     

Seismicity and structural investigations of the Romanian Vrancea Region: Evidence for azimuthal variations of p-wave velocity and poisson's ratio

A study of the shallow and intermediate depth seismicity of the Romanian Vrancea region in the period 1964–1981 has been performed. The seismic events have been relocated by a standard location procedure using a regional velocity model. From the temporal and spatial distribution of the seismic activity, aspects of the seismicity before the large March 4, 1977 earthquake are treated, in particular the seismic gap in space and time prior to this event, found by Mârze (1979), which is critically discussed and revised. The concept of the precursor time/magnitude relationships of different authors is applied and its validity to the Vrancea region assessed. The hypocentral distribution shows that the intermediate depth seismic activity is confined to a small volume with dimensions of only some tens of kilometers. The results are interpreted in terms of the tectonics of the region. From an analysis of the travel-time residuals at different local stations, evidence for lateral velocity heterogeneities beneath the region is obtained e.g. a high velocity zone southeastwards of the Carpathian chain. Finally mean ratios, (i.e. Poisson's ratios), for various stations are calculated from P- and S-wave travel times. They show azimuthal variations of up to 6% for stations within the area where the intermediate seismic activity occurs in comparison with the station Focsani, situated eastwards in the Carpathian foredeeps. All these results are compatible with the plate tectonic concept for the Vrancea region, that is the subduction of an oceanic lithospheric slab under the Carpathian mountain arc, giving rise to such a highly active seismic zone.     

The gold fraction of the Witwatersrand conglomerates from the Orange Free State goldfield (South Africa)

An investigation of the gold fraction of the Witwatersrand conglomerates from the Orange Free State goldfield (South Africa) has confirmed that the bulk of this metal is concentrated within the bottom parts of the reefs, mostly near the footwall contacts. This, together with other criteria, suggests that the distribution of the metal was governed by sedimentary processes. On the other hand, the shape of the gold grains, as well as the general fabric of the gold-enriched layers, often lack conspicuous sedimentary characteristics. Subsequent metamorphism has obviously produced marked structural modifications and a limited textural rearrangement of the gold and its host-rock, thus obliterating to a certain degree the primary alluvial character of the metallization. The large-scale reconstitution of the gold fraction was effected either by a mechanical reshaping of the malleable and ductile gold particles, or by an intermediate solution process followed by reprecipitation. These processes took place more or less in situ. Accordingly, the original sedimentary distribution pattern was not profoundly affected, whereas the individual allogenic gold grains were transformed into authigenic ones or sometimes even into major gold aggregates. Electron-probe microanalysis established that the gold contains on the average between 9.9 and 12.4% silver, which is also suggestive of an alluvial origin. In spite of some uncertainties arising from the camouflaging effect of pseudohydrothermal reconstitution processes and from the absence of hydraulic equilibrium between gold and several demonstrably detrital heavy minerals, an integration of all available data strongly supports a modified placer theory for the origin of the Witwatersrand gold.

---

Zusammenfassung Eine Untersuchung der Goldführung von Konglomeraten des Oberen Witwatersrand-Systems (Oranje-Freistaat-Goldfeld, Südafrika) zeigt, daß die Verteilung des Metalls eindeutig durch sedimentäre Faktoren gesteuert worden ist, obwohl die Form der einzelnen Goldkörnchen sowie ihre Lagebeziehungen zueinander und zu den übrigen Mineralien nicht in jedem Falle mit gleicher Eindeutigkeit für ein sedimentäres Gefüge sprechen. Für diesen scheinbaren Widerspruch ist die Metamorphose verantwortlich, der die Lagerstätte im Laufe ihrer späteren geologischen Geschichte ausgesetzt war und bei der es zu einer teilweisen Umlagerung der Lagerstättensubstanz kam. Davon wurde auch die Goldfraktion betroffen, die dementsprechend heute kaum noch in ihrer ursprünglichen detritischen Form vorliegt, sondern meist pseudohydrothermale Aggregate bildet. Die Umkristallisation des Goldes erfolgte dabei meist in situ, so daß das ursprüngliche sedimentäre Verteilungsschema ausgezeichnet erhalten geblieben ist. — Der Silbergehalt des Goldes konnte mit 9.8–12.4% bestimmt werden, was gleichfalls für eine detritische Herkunft des Metalls spricht. Berücksichtigt man die späteren Umlagerungsvorgänge, dann bereitet der erzmikroskopische Befund einer Deutung der Konglomerate als Seifen keine Schwierigkeiten, sondern spricht mit überwältigender Wahrscheinlichkeit für eine Seifenumlagerungshypothese.

     

Evolution of photoautotrophy and early atmospheric oxygen levels

Current photochemical models suggest that oxygen levels in the prebiological atmosphere were extremely low, most probably remaining in the range 10⁻⁸–10⁻¹⁴ PAL (present atmospheric level). It is, therefore, reasonable to assume that only life processes were able to overwhelm these minor O₂-pressures, with free oxygen resulting from the reduction of carbon dioxide to the carbohydrate level during photoautotrophic carbon fixation using water as an electron donor (

Full-size image (1K)

). It is by now well established that reduced (organic) carbon is a common constituent of sedimentary rocks from the very start of the geological record 3.8 Ga ago. Both direct assays and inferences derived from a carbon isotope mass balance suggest that the C_org-content of Archaean sediments was not basically different from that of geologically younger rocks. This poses the problem of the existence 3.5 Ga ago of an oxidation equivalent of such a formidable ancient C_org-reservoir which, depending on the model adopted for the growth of the sedimentary mass through time, might have amounted to between 20 and 100% of the present one. Low atmospheric oxygen pressures in the Early Precambrian that are inferred from retarded oxidation reactions, notably in the ancient continental weathering cycle, are likely, therefore, to indicate extremely rapid processes of oxygen consumption in other parts of the system (e.g., hydrosphere) rather than the general absence of photosynthetic oxidation equivalents during this time.     

Whole-bird models for the magnetic cleansing of oiled feathers

Iron powder, a promising dry-cleansing agent for oiled feathers where both the contaminant and the cleansing agent may be harvested magnetically, has been tested on the plumage of whole-bird models. The breast and back plumage of Mallard Duck (Anas platyrhynchos) and Little Penguin (Eudyptula minor) carcasses were patch-contaminated with commercial-grade engine oil, three different crude oils, and an oil/seawater emulsion. The plumage was then subjected to a magnetic cleansing protocol. The contaminant removal was assessed gravimetrically and was found to reflect the outcomes for a previously reported in vitro study using feather clusters. Between 92-98% of the contaminants, and effectively all of the cleansing agent, were removed from the feathers.